The present invention relates to a pressure screen for separating a liquid mixture containing undesired coarse contaminants, comprising a screen housing forming a cylindrical screen chamber, an inlet member for supplying the liquid mixture to be separated to the screen chamber, and at least one outlet member for discharging coarse contaminants from the pressure screen. In particular, the present invention relates to such a pressure screen designed for separating fiber suspensions, such as paper pulp suspensions. The coarse contaminants possibly occurring in such a suspension usually comprise scrap particles in the form of stone particles and metal pieces, which can damage and in the worst case cause breakdown of various elements located in the screen chamber, such as a rotor and/or a screen basket.
In a traditional pressure screen with a rotor and a screen basket, the suspension is supplied through the inlet member tangentially into the cylindrical screen chamber to form a vortex. As a result, scrap particles rotating in the vortex are subjected to centrifugal forces that pull the scrap particles radially outwardly in the screen chamber, so that the scrap particles at best pass through the outlet member without interfering the rotor in the screen chamber. However, it has been proved that the velocity of the incoming fiber suspension into the screen chamber often is not sufficient to cause sufficient separation of all the scrap particles from the suspension to the outlet member. Therefore, the rotor and/or the screen basket often are damaged by heavy scrap particles, sometimes to such an extent that the pressure screen has to be taken out of order for repair.
A possible solution to the above problem of insufficient separation of scrap particles could be to increase the flow velocity of the fiber suspension to thereby increase the centrifugal forces acting on the scrap particles. However, this is not any practicable way, since a higher inlet velocity of the fiber suspension gives unacceptably high pressure losses across the screen and in addition a greater wear.
One object of the present invention is to provide an improved pressure screen having an efficient scrap separation property.
This and other objects are obtained by the pressure screen stated initially, which is characterized in that it further comprises an inlet housing forming a cylindrical inlet chamber having a diameter which is less than the diameter of the cylindrical screen chamber and being in fluid communication with the screen chamber. The inlet member is positioned on the inlet housing for supplying the liquid mixture substantially tangentially into the cylindrical inlet chamber and the outlet member is positioned on the inlet housing for discharging the coarse contaminants from the inlet chamber.
Since the incoming suspension will form a relatively narrow vortex in the cylindrical inlet chamber, the diameter of which is less than the diameter of the cylindrical screen chamber, heavier contaminants, such as scrap particles in the form of stone particles and metal pieces, will be subjected to stronger centrifugal forces that efficiently separate the scrap particles outwardly in the inlet chamber to the outlet member, so that the scrap particles are prevented from entering the screen chamber. As a result, the screen according to the present invention provides a safe pre-separation of scrap particles before the suspension is subjected to the main separation process in the screen chamber where a rotor and a screen basket normally are situated.
Preferably, the diameter of the cylindrical inlet chamber is at least 10% less than the diameter of the cylindrical screen chamber.
In an embodiment of the present invention, the cylindrical inlet chamber communicates with the cylindrical screen chamber by means of an axial passage between them, so that scrap particles rotating in the inlet chamber do not tend to enter the screen chamber. Preferably, the inlet housing is directly connected to the screen housing and situated below the screen housing, so that scrap particles in the inlet chamber are prevented by gravity from entering the screen chamber.
The axial passage may have a circular cross-section with the same diameter as the cylindrical inlet chamber and the cylindrical screen chamber may extend in parallel with the cylindrical inlet chamber, and preferably be coaxial thereto, which results in a simple design of the screen.